In venous blood sampling, a needle is inserted into a vein to collect a sample of blood for testing.
Peripheral veins, typically the antecubital veins, are the usual sites for venous blood sampling.
Ultrasound guidance, when equipment and trained personnel are available, can facilitate blood sampling from deep, nonpalpable veins.
(See also Vascular Access.)
Indications for Venous Blood Sampling
Need for a venous blood sample for laboratory study
Contraindications to Venous Blood Sampling
Absolute contraindications
None
Relative contraindications
Infection or hematoma at a prospective venipuncture site
Injured or massively edematous extremity
Thrombotic or phlebitic vein
Intravenous catheter distal to a prospective venipuncture site (may affect laboratory results if IV fluids or drugs are infusing distal to the venipuncture site)
Ipsilateral mastectomy
In the above situations, use another site (eg, the opposite arm) for venipuncture.
Coagulopathy is not a contraindication, but sites must be compressed for a longer time after venipuncture.
Complications of Venous Blood Sampling
Complications are uncommon and include
Local infection
Arterial puncture
Hematoma or bleeding
Damage to the vein
Nerve damage
Vasovagal syncope
Equipment for Venous Blood Sampling
Nonsterile gloves (sterile gloves if blood cultures are being obtained)
Tourniquet, single-use
Needle system (eg, needle and syringe, or needle and vacuum tube, typically 21-gauge needles for adults; 22- or 23-gauge for neonates, small children, and sometimes older patients)
Blood-collection tubes and blood-culture bottles, as appropriate
Dressing materials (eg, tape, gauze, bandages)
Optional equipment includes
Vein-finder device (eg, infrared vein viewer, ultrasonography device)
Additional Considerations for Venous Blood Sampling
Latex hypersensitivity: Use latex-free gloves and tourniquet.
Efficient, expeditious performance of the procedure is required to avoid prolonged tourniquet placement and blood stasis, which can cause artifactually abnormal laboratory results (eg, hemolysis and hyperkalemia).
hyperphosphatemia, or hyperuricemia) and also to avoid skin irritation.
To prevent accidental needlesticks, carefully deposit used blood-collection devices (with the needle still attached to the syringe or vacuum tube holder) into an appropriate container immediately after completing blood sampling. Do not recap standard (ie, nonsafety) needles prior to disposal unless a container is not immediately available.
Relevant Anatomy for Venous Blood Sampling
The median cubital and cephalic veins are preferred for blood sampling, but other arm and hand veins may be used.
The cephalic vein is located on the lateral (radial) side of the arm, and the basilic vein is located on the medial (ulnar) side. These veins extend through and branch within the antecubital fossa, creating the large antecubital and proximal forearm veins.
The brachial artery (blood supply to the forearm and hand) lies deep to the basilic vein in the upper arm and bifurcates into the radial and ulnar arteries in the antecubital fossa or proximal forearm. Caution must be taken to avoid arterial puncture.
Leg veins (eg, dorsal digital veins and the greater saphenous vein inferoanterior to the medial malleolus) or external jugular veins may be used if upper extremity veins are inaccessible.
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Positioning for Venous Blood Sampling
Patient should be seated with back supported or, if they are anxious or have a history of vasovagal reaction, supine.
Support the supinated forearm (or other chosen site) on a comfortable surface.
To access the external jugular vein, place the patient in Trendelenburg position with the head tilted slightly contralaterally.
Step-by-Step Description of Venous Blood Sampling
Identify and prepare the site
Do a preliminary inspection (nonsterile) to identify a suitable vein: Apply a tourniquet, have the patient make a fist, and palpate using your index finger to locate a large-diameter vein that is nonmobile and has good turgor.
To help distend and locate veins, tap a potential site with your fingertips. It may help to allow the arm to hang down, increasing venous pressure. Use a vein-finder device if a suitable vein is not readily seen or palpated.
After identifying a suitable cannulation site, remove the tourniquet.
Apply anesthetic if it is being used and allow adequate time for it to take effect (eg, 1 to 2 minutes for gas injector, 30 minutes for topical).
Cleanse the skin site with antiseptic solution, beginning at the needle-insertion site and making several outwardly expanding circles.
If blood is being obtained for blood cultures
Obtain the blood sample
Try to access the vein efficiently and collect the blood sample within 30 seconds after tourniquet placement. Do not leave the tourniquet on for > 1 minute.
Reapply the tourniquet proximal to the selected insertion site. Do not have patients make a fist or let their arm hang down during the blood sampling, because these maneuvers may cause various erroneous laboratory values (eg, increased potassium, lactate, phosphate).
Palpate with your gloved finger to locate the middle of the target vein.
Apply gentle traction to the vein distally using the thumb of your nondominant hand to prevent the vein from moving. Traction may not be necessary for larger veins of the forearm or antecubital fossa.
Tell the patient that the needlestick is about to happen.
Insert the needle proximally (ie, in the direction of venous blood flow), with the bevel facing up, along the midline of the vein at a shallow angle (about 10 to 30 degrees) to the skin.
Blood will appear in the needle hub (called a blood flash ) when the needle tip enters the lumen of the vein. Stop advancing the needle, lower the needle to better align it with the vein, and advance it into the vein an additional 1 to 4 mm, to ensure that it stays in position during blood collection.
If no flash appears in the hub after 1 to 2 cm of insertion, withdraw the needle slowly. If the needle had initially passed completely through the vein, a flash may now appear as you withdraw the needle tip back into the lumen. If a flash still does not appear, withdraw the needle almost to the skin surface, change direction, and try again to advance the needle into the vein.
If rapid local swelling occurs, blood is extravasating. Terminate the procedure: Remove the tourniquet and the needle and apply pressure to the puncture site with a gauze pad (a minute or 2 is usually adequate unless the patient has a coagulopathy).
Keep the needle motionless.
Begin to withdraw the blood sample and, when blood begins to flow, remove the tourniquet.
When using vacuum tubes, push each tube fully into the tube holder, use care to avoid dislodging the needle from the vein. Fill multiple collection tubes in the proper sequence.* After removing a tube from the holder, gently invert the tube 6 to 8 times to mix the contents; do not shake the tubes.
When using a syringe, pull back on the plunger gently to avoid damaging the blood cells or collapsing the vein.
When blood collection is complete, gently hold a folded gauze square at the venipuncture site with your nondominant hand, and in one motion remove the needle and immediately apply pressure to the site with the gauze. Remove the tourniquet if you did not do so earlier.
Have the patient or an assistant continue to apply pressure to the site.
If you used a syringe to collect the blood, now transfer samples to collection tubes and bottles;* either insert the needle directly into the tops of the vacuum tubes, or remove the needle and attach a vacuum tube holder to the syringe. Do not inject blood into vacuum collection tubes; allow the vacuum to draw the blood into the tube. After blood has been added to a tube, gently invert the tube 6 to 8 times to mix the contents; do not shake the tubes.
Deploy safety cover over the exposed needle. Deposit used blood-collection devices (with needles) into a sharps container. Do not recap non-safety needles prior to disposal unless a sharps container is not immediately available.
Dress the site with gauze and tape or a bandage.
* When multiple blood tests are to be done, blood should be allocated to the collection tubes in a proper sequence; first cultures, then tubes with anticoagulant, and then others.
Note that the rubber tops of blood-culture bottles must be properly disinfected prior to introducing the blood sample (eg, by scrubbing each top with separate 70% alcohol wipes for 30 seconds and allowing it to air-dry).
Aftercare for Venous Blood Sampling
Recheck the site after a few minutes to verify the absence of hemorrhage/hematoma.
Warnings and Common Errors for Venous Blood Sampling
Use only mild tension when applying the tourniquet; it is a venous, not an arterial, tourniquet. Remember to remove the tourniquet after the blood draw.
Take care not to puncture too deeply and go through the vein.
If the vein is not entered, do not try to reposition the needle by moving the tip to one side or another; this can push the vein out of the way and also damage tissue. Instead, withdraw the needle partway before changing the angle and direction of insertion.
It is no longer recommended to flex the elbow after antecubital venipuncture; this actually increases hematoma formation.
Tips and Tricks for Venous Blood Sampling
Butterfly needles attached to a syringe may be preferred for difficult venipuncture (eg, small veins in neonates, fragile veins in older adults).
Well-fitting gloves make palpation of the vein easier.
Instruct patients to look away from the equipment and the procedure to help prevent a vasovagal episode.
